The present invention relates to a method for monitoring the pressure of motor vehicle tires.
Various apparatuses and methods for monitoring the pressure of motor vehicle tires are known from modern vehicles or from the related art. For example, U.S. Pat. No. 5,694,111 describes a tire pressure indicator in which the pressure data detected by a pressure sensor is compared with a predetermined operating pressure range. An abnormal state of the air pressure is indicated as a function of the comparison result.
German patent document DE 101 36 831 A1 discloses a method and an apparatus for monitoring the pressure of pneumatic tires, and for detecting an abnormal state of the pressure and/or the temperature of the individual tires of the vehicle. The pressure and/or the temperature of the individual tires on the vehicle are/is monitored, and it is possible to distinguish between individual pneumatic tires on the vehicle, in order to verify which tire is in an abnormal state. It is also known from German patent document DE 101 36 831 A1 to have a monitoring apparatus controlled by operation of the vehicle key, or for the monitoring apparatus to be used to compare the present state of the tire with the state of the tire when the motor vehicle was started. If the difference between the air pressure values is greater than a predetermined value, the tire is classified as being in a slow leakage state.
German patent document DE 101 05 641 A1 discloses a tire pressure monitoring system which combines a direct measuring system including a pressure sensor with an indirect measuring system based on a wheel rotation speed sensor system. In the case of this tire pressure monitoring system, the indirect measuring system is calibrated by using the detected tire pressure values of the direct measuring system. There is no need for manual calibration of the direct measuring system, for example, by manually inputting tire pressure values and subsequent operation of a calibration key.
A method for monitoring the pressure of motor vehicle tires during which a tire pressure value indicative of the tire filling pressure is determined is known from modern motor vehicles. The determined tire pressure value is compared with a stored nominal value, and the comparison result is used to deduce whether the motor vehicle tire is at an incorrect tire pressure.
Against the background of this prior art, an object of the present invention is to specify a method for monitoring the pressure of motor vehicle tires, which is more reliable and can be carried out more easily and more conveniently for the driver.
This and other objects and advantages are achieved by the method according to the invention, in which a tire pressure value that describes the tire filling pressure is determined, in order to monitor the pressure of motor vehicle tires. The determined tire pressure value is compared with a stored nominal value. The comparison result is used to deduce whether the motor vehicle tire is at an incorrect tire pressure, in particular, a low tire pressure. In the case of a characteristic change in the tire pressure, the stored nominal value is replaced by a new nominal value, with the determined tire pressure value being used to determine the new nominal value. Various predetermined criteria can be checked in order to determine whether a characteristic change has occurred in the tire pressure. An advantage of the method according to the present invention is that it can be carried out fully automatically, while taking into account any change which may have occurred deliberately in the tire air pressure, for example, as a result of a tire being filled or as a result of a tire change. In particular, in a situation such as this, there is no need for initialization, the inputting of values or calibration by the driver. This means that the method according to the present invention is not only more reliable but is also more convenient.
In order to take account of the relationship between the tire pressure and the tire temperature or the temperature of the tire filling means, a tire temperature value can additionally be determined and can be used for correction or normalization of the tire pressure. In the same way, the nominal value can be normalized with respect to a temperature value, or can be stored as a temperature-dependent value. Overall, the method according to the present invention can be carried out in such a way that the tire temperature value or the tire temperature values are taken into account for all pressure values. In particular, all pressure values may be temperature-compensated.
In one refinement of the method according to the present invention, the determined tire pressure value is compared with a stored comparison pressure value, which was determined at an earlier time, with the tire temperature value being taken into account, to determine whether a characteristic change has occurred in the tire filling pressure. A characteristic change in the tire filling pressure occurs in particular when the difference between the determined tire pressure value and the stored comparison pressure value is greater than a predetermined threshold value.
In another refinement of the method, a characteristic change in the tire filling pressure occurs when the difference between the determined tire pressure value and the stored comparison pressure value is greater than a predetermined threshold value for at least two wheels. This threshold value may be chosen to be relatively small, for example, between 0.1 bar and 0.4 bar. The pressure differences may be temperature-compensated.
In a further refinement of the method, a characteristic change in the tire filling pressure occurs only when the vehicle has been stopped or restarted between the time of the determined tire pressure value and the earlier time of storage of the stored comparison pressure value.
In one embodiment of the method, the determined tire pressure value is also subjected to a plausibility check, with a new nominal value being stored only when the determined tire pressure value is classified as being plausible.
In one refinement of this, a tire pressure value such as this is classified as being plausible only if the difference between this first tire pressure value and a further tire pressure value associated with the same vehicle axle and the opposite vehicle side is less than a predetermined threshold value, for example less than 0.4 bar.
Alternatively or additionally, it is possible for plausibility to check whether all of the determined tire pressure values are above a predetermined threshold value. By way of example, a tire pressure value is classified as being plausible only when all of the determined tire pressure values are greater than 1.6 bar.
In a further refinement, a tire pressure value is classified as being plausible only when the determined tire pressure value associated with the rear vehicle axle is greater than the mean value of the determined tire pressure values associated with the front vehicle axle, minus a predetermined constant, in which case, in particular, the constant may be equal to zero.
As a further criterion for plausibility of a determined tire pressure value, it is possible to check whether the difference between the determined tire temperature and an ambient temperature is less than a predetermined threshold value. By way of example, a tire pressure value is classified as plausible only when the difference between the determined tire temperature and a determined ambient temperature is less than 40° C.
The plausibility conditions described in the above paragraphs can additionally be linked to a time criterion. For example, a tire pressure value is classified as being plausible only when the respective plausibility conditions are satisfied at least for a time period associated with them, e.g., for at least five minutes. A characteristic change in the tire filling pressure occurs in particular when the tire filling pressure of one or more tires on the vehicle has been reset by the driver or by someone else, and/or the tires have been filled with air, or one or more wheels on the vehicle have been replaced. Other criteria which indicate such a deliberate change in the tire air pressure of a vehicle tire may be checked in addition to or as an alternative to the criteria mentioned above, and may be used as a decision criterion or plausibility criterion.
The method according to the present invention may be carried out in order to monitor the pressure of all of the vehicle tires. If the method is carried out for a plurality of vehicle tires, a separate nominal value may be associated with each tire individually, or with each tire pair arranged on one axle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.